Low capacity filament transformer system



March 16, 1943. J. 1.. FINCH LOW CAPACITY FILAMENT TRANSFORMER SYSTEM 2 Sheets-Sheet 2 Filed March 15, 1940 Sm l n 3 Z wm V R.H Y. .EF 0 T H I Patented Mar. 16, 1943 LOW CAPACITY FILAMENT TRANSFORMER SYSTEM James L. Finch, Patchogue, N. Y., assignor to Radio Corporationof America, a corporation of Delaware Application March 15, 1940, Serial No. 324,035

Claims. (Cl. .250--2'7)v This invention relates to short wave radio frequency generators and amplifiers which employ thermionic devices and particularly to such devices wherein the cathodes are heated by alternating current and wherein the anodes are maintained at an appreciably generated or amplified wave voltage.

A system according to this invention will be found particularly advantageous when anodes of the thermionic devices are fitted with bulky metallic fin devices, over which air is blown for the purpose of cooling them. In such devices, the anodes of the thermionic devices can be grounded with respect to the electric waves which are being generated or amplified.

In the prior art, it has been the usual practice to ground the cathode and to subject the anodes to the electric wave voltages. The high capacity of the anodes to ground and to other objects when bulky metallic cooling fins are used is found objectionable. The resultant high capacity may limit the maximum usable frequency to a materially lower value than would be otherwise possible. Further, when the anode was grounded and accordingly both the grid and the cathode of the thermionic devices subjected to high voltages, it was necessary to supply heating current to the cathode through chokes or tuning coils. Such a method proved to be a handicap in many cases. One detrimental effect was that it made it more difficult to change the frequency of the tuned circuits involved. Another detrimental factor was that the chokes, etc., introduced losses both in the high frequency wave voltage circuit and in the heater circuit. This method of supplying cathode heater power has generally been found impractical at radio frequency waves below twenty megacycles.

It is an object of my invention to improve such a system by employing a transformer in which the windings are isolated from ground and by the introduction of an improved radio frequency circuit which employs tuning means between the grid and the anode of the thermionic device for the purpose of preventing regeneration due to grid-anode capacity and for the purpose of preventing excitation power from feeding through to the output due to the last mentioned capacity.

This invention will best be understood by referring to the accompanying drawings, in which:

Fig. 1 is a schematic circuit arrangement of the system of this invention;

Fig. 2 is a section of the improved filament transformer;

Fig, 3 is a plan view of Fig. 2; and

Fig. 4 is a modified sectional view similar to that of Fig. 2, except that both the primary and secondary are insulated from the core by an air p.-

Referring now in detail to the drawings, Fig. 1 shows a circuit diagram of a radio frequency amplifier having a range up to twenty megacycles in frequency. The thermionic triode device is of a type similar to an RCA 893 tube and is indicated as 1 having an anode 2, grid 3 and filament 4. The filament is shown as consisting of three separate strands connected together at their lower ends. The RCA 893 thermionic triode actually has six such strands connected together at their lower ends, so that in the figure each line is representative of two strands in parallel. A metallic fin assembly, preferably of copper, surrounds the anode 2 for cooling the same. This cooling assembly consists of a hub 6 of solid copper into which the anode is soldered. A plurality of fins I extend radially out from hub 6. A blast of air is blown between the fins, as indicated by the arrows. The radio frequency circuit employs a tuning means between the grid and anode for the purpose of preventing regeneration due to the grid-anode capacity and for the purpose of preventing excitation power from feeding through to the output due to the grid-anode capacity. This tuning means consists essentially of an inductance coil 8 and a trimming condenser 9, which also includes bias bypass condenser I0 and a plate bypass condenser ll having a common ground connection between the two. The input circuit is disposed between the grid and filament of the thermionic tube. This circuit is tuned by making the reactance of coil l2 equal to the combined reactance of the grid-to-filament capacity, and the tuning condenser l3. The source of excitation power is represented by a generator l4. Generator l4 supplies current to coil I5 which is inductively coupled to coil 12 and I have found it advisable to shield against capacitive coupling between these two circuits by means of means of astatic shield IS. The output circuit is connected between the filaments and the anode and comprises tuning coil ll, tuning condenser N3, the common ground connection, and the bypass condenser II. The reactance of coil I1 is so determined that with the filament-to-ground capacity, the filament-to-anode capacity, and the capacity of tuning condenser I8, the output circuit is resonant to the operating frequency of the system. The output circuit is coupled in some suitable manner to the load. In this case,

connected to the anode assembly. Thus, the anode is subjected to the full voltage of the plate supply. However, it is subjected to negligible.

radio frequency voltage, since any such voltage is bypassed to ground through condenser l'l The anode current flows to the filament through the:

individual transformer windings and through tuning coil l1, thus returning to the negative side of the anode supply through the common ground connection. Bias voltage for the amplifier 'systern is introduced from the source indicated as C through grid-anode tuning inductance 8 to the grid 3. The return circuit for the grid current is through the filaments, through tuning coil 57 and the common ground connection to the positive end of the grid bias source. The radio frequency connections are made to the filament by means of bypass condensers 28, shown located adjacent to the low capacity transformer assembly 20.

Referring now in detail to Figs. 2 and 3, it will be noted that this construction differs from the ordinary filament transformer, as known in the prior art, in that the secondary windings are isolated-from the iron core and from the primary windings by an air gap, that is, there is provided a space between the core and the coil winding in which there is no metallic substance or solid dielectric material. Such a construction insures a low capacity between the secondary windings and other adjacent objects. The secondary windings ZB'and 21 are shown supported on angles 29 and 30 which in turn are supported by four insulators 3l,.3l a, 32 and 32a. The iron core 2! is indicated as being substantially circular in section. This allows the minimum spacing for a given impressed voltage. The circular core is constructed by having the central laminations larger than the outermost laminations, that is, each lamination on each side of the center line decreases proportionally in size on each side of the central line, thus providing a core essentially circular in crcss-section The secondary windings are armored with a nonmagnetic metal 35 which protects the insulation of the individual conductor turns from the radio frequency field. It also protects the insulation from damage in case of an arc between the secondary windings and the core. This armor 35 is connected to the common point of the three secondary windings as indicated in the circuit diagram of Fig. 1. Located directly below each individual secondary Winding is a primary winding 22, 23 and 24. Core 2| is mechanically secured to a base plate 36 by means of a plurality of strap angles 31 secured to plate 36 by means of screws 38. Bypass condensers 28 are mounted on angle 29 adjacent to each secondary winding and connected thereto by leads 39 and Ml.

Fig. 4 shows a further modification from that of Figs. 2 and 3 in which the core is isolated by an air gap from the primary in the same manner as shown and previously described in which the secondary was isolated. The core in this casewill also. be supported by a strap secured to insulators 3| and 32. This construction will further decrease the capacity.

While I have described my invention as related to a system using three-phase filaments and have shown the same as connected with three-phase transformers, this invention applies equally well to single phase filament supply and three single phase transformers can be used for a three-phase supply.

What is claimed is:

l. A high frequency wave system comprising a thermionic device having a heated cathode, an anode and a grid, a source of high frequency wave. energy coupled to said grid and said cathode, said ancde beingjmaintained at substantially ground potential for said wave energy, an output circuit coupled to said cathode and said anode and. means coupled to said cathode for heating said cathode, said means including a transformer having a primary and secondary and a core, the secondary of said transformer being so spaced from said core and primary that a high impedance for said high frequency wave energy is presented between said cathode and ground.

2. In a high frequency generating system for radio frequency according to claim 1, with the characteristic feature that an air space is provided between the secondary and the core.

3. In a high frequency generating system according to claim 1 with the characteristic feature that an air space is provided between the secondary and the core and the primary and the core and that the secondary and the core are supported on insulators.

4. In a high frequency generator according to claim 1 with the characteristic feature that the transformer windings are encased in a metallic shield, said shield being connected. to one side of the secondary winding.

5. A radio frequency transmitter as claimed in claim 1 with the characteristic feature that the core of said transformer has a circular crosssection.

JAMES L. FINCH. 

